n, m = map(int, input().split())

if n > m and n % 2 == 1:
    print(-1)
elif n > m and n % 2 == 0 and n // 2 > m:
    print(-1)
elif n > m and n % 2 == 0 and n //2 < m:
    print(2)
elif n <= m:
    print(1)